The present application claims priority to Japanese Application(s) No(s). P2000-271301 filed Sep. 7, 2000, which application(s) is/are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a semiconductor emission apparatus serving as a light source for optical recording systems, laser printers, laser displays and the like, more specifically to a semiconductor emission apparatus comprising a semiconductor light-emitting device with a short-wavelength such as a blue-colored semiconductor laser inside its package.
2. Description of the Related Art
In general, in a semiconductor emission apparatus such as LDs (Laser Diodes), laser oscillation is performed by amplifying light using a Fabry-Pe""rot resonator in which light goes back and forth between two mirrors placed parallel to each other. At this time, the degree of parallel between the two mirrors is extremely important. For example, a gallium arsenide (GaAs) substrate, which is mainly used for a laser for CDs (Compact Discs), MDs (Mini Discs) or DVDs (Digital Video Discs), is extremely excellent in the cleavage characteristic and the degree of parallel between the end faces formed by cleaving becomes uniform in atomic order (by 5 angstrom unit). Therefore, if the end face of a resonator is formed using a GaAs substrate, an excellent mirror of the resonator can be obtained.
On the contrary, sapphire is used for a substrate of a device in a semiconductor emission apparatus formed by laminating Group III-V nitride semiconductor film. A sapphire substrate is capable of growing a semiconductor film having an excellent crystalline characteristic. On the other hand, the sapphire substrate has a poor cleavage characteristic so that the degree of parallel between the end faces formed by cleaving becomes notably poor. Therefore, research and development have been proceeded on substrate materials with excellent cleavage characteristics, which can be substitutes for sapphire. Specifically, silicon carbide (SiC), gallium nitride (GaN) and the like have been studied.
Among those mentioned, a GaN substrate is advantageous as a substrate for a semiconductor laser since a GaN semiconductor, which is the same kind of the substrate, can be easily grown thereon with a high crystalline characteristic, and also impurities can be added to the substrate itself giving it electrical conductivity. However, the semiconductor laser using the GaN substrate has a problem that the beam pattern (Far Field Pattern; FFP) is not perfectly unimodal.
When using the semiconductor laser for light source of systems such as optical disk systems or laser-beam printers, it is generally desirable that the FFP is unimodal. For example, spikes and the like generated at the end of the beam, which cannot be ignored, become noises on the system application. The reason is that if there are a plurality of pits when concentrating light emitted from the semiconductor laser, two spots are formed in the optical disk and two dots are formed in the laser-beam printer. In practice, the unimodal FFP is used for 780 nm band and 650 nm band lasers (for example, SLD231, 233, 134, 1133) for CDs, MDs, DVDs, and laser printers, which have already been on the market.
Therefore, control of the beam pattern becomes the subject. It will be described by referring to FIGS. 1A to 1D. FIG. 1A shows a basic structure of a semiconductor laser having a configuration in which a cladding layer 212, an active layer 213, and a cladding layer 214 are laminated on a GaN substrate 211. The distribution of index of refraction of the laser in the laminating direction is as shown in FIG. 1B. The light is confined in the active layer 213 by the two cladding layers 212 and 214. However, the mode confined in the active layer 213 leaks to the cladding layer 212 and decreases exponentially as getting closer to the GaN substrate 211. When the waveguide mode of the active layer 213 cannot be sufficiently decreased due to the insufficient layer thickness of the cladding layer 212 and reaches the substrate, light leaks inside the GaN substrate 211. The light distribution in this case is as shown in FIG. 1C. When the light guided by such light distribution is emitted from the semiconductor laser, the FFP does not become unimodal but has two spike-like sharp peaks as shown in FIG. 1D. In other words, in the waveguide mode in this case, there are two waves such as the mode component in the active layer 213 and the leak component in the GaN substrate 211. The latter forms a spike in the FFP.
In general, there are two methods considered for suppressing the generation of the spike-like peaks as described. One is to reduce the index of refraction of the cladding layers especially the cladding layer 212 on the substrate side and the other is to thicken the layer thickness. In the Group III-V nitride semiconductor laser using the GaN substrate, it is necessary to reduce the index of refraction by increasing the amount of aluminum (Al) in the cladding layer, which is AlxGa1xe2x88x92xN layer, or to increase the layer thickness. In order for the AlxGa1xe2x88x92xN layer to effectively confine (that is, coefficient of confinement is 0.02 and more) the light inside the active layer, it is necessary to increase the amount of aluminum (Al) to the composition ratio x greater than 0.06. On the other hand, the critical film thickness determined by the elastic constant and differences in the lattice constant between the GaN and the AlxGa1xe2x88x92xN decreases reciprocal to the amount of aluminum (Al). Therefore, when the index of refraction is decreased, cracks are easily generated so that it becomes difficult to increase the film thickness. Considering the values of properties of the GaN and the AlxGa1xe2x88x92xN ,which is distinctive to the materials, such as the lattice constant, the index of refraction, and the elastic coefficient, it is extremely difficult to satisfy the two conditions at the same time, that is, to confine the light and have the sufficient critical film thickness even by changing the value of properties by designing the materials.
As described, in the Group III-V nitride semiconductor laser using the GaN substrate, it is difficult to suppress generation of the spike-like peaks by the methods of the related art in which the values of properties is controlled by designing the materials.
The invention is designed to overcome the foregoing problems. The object is to provide a semiconductor emission apparatus capable of emitting only unimodal beams by a simple structure without designing materials.
A semiconductor emission apparatus of the invention comprises a semiconductor light-emitting device oscillating an optical beam; and an enclosing member (package) for enclosing the semiconductor light-emitting device while having a light-leading window for emitting a beam oscillated from the semiconductor light-emitting device, and for shielding unnecessary components among the components of the beam. The invention is especially effective in the case where the semiconductor light-emitting device has a structure in which Group III-V nitride semiconductor film including at least one kind of element selected from the group consisted of aluminum (Al), gallium (Ga), indium (In) and boron (B) is laminated on a substrate made of gallium nitride (GaN).
In a semiconductor emission apparatus of the invention, the unnecessary components among the beam components oscillated from the semiconductor light-emitting device is shielded by the enclosing member (package) itself. As a result, only the necessary unimodal beam component is emitted from the light-leading window.
Other and further objects, features and advantages of the invention will appear more fully from the following description.